1. Field of the Invintion
The present invention relates to a thermosensitive stencil sheet and a thermosensitive stencil plate, and to a thermal stencil plate making and stencil printing method therefor.
2. Discussion of the Related Art
Currently, thermal stencil plate making and stencil printing methods (hereinafter referred to as stencil printing methods) are well known in which a thermosensitive stencil sheet, which includes a thermoplastic resin film formed overlying an ink permeable substrate, is heated imagewise with a thermal printhead to form imagewise perforations in the resin film, i.e., to form a stencil plate, and then printing ink is allowed to pass through the perforations to form an image on a receiving material such as paper. However, the density of the perforations of these stencil printing methods, which depends on the density of the heating elements of the thermal printhead used for the imagewise heating, is 600 dpi (dots per inch) at the most under the present conditions, and therefore the resolution of the resultant print image is not good. In addition, the printing methods have another drawback in that the thermoplastic film tends to adhere to the thermal printhead and thereby desired perforations cannot be formed in the stencil sheet because of insufficient heat conduction of the heat elements, resulting in occurrence of omissions in the resultant print image. In attempting to improve these drawbacks, thermal stencil printing methods have been proposed in which an electromagnetic wave such as laser light. digitally irradiates a thermoplastic resin film of a stencil sheet to form imagewise perforations.
Japanese Laid-Open Patent Publication No. 61-229560 discloses a method for forming a stencil plate in which a laser beam which is modulated according to image patterns scans on a thermoplastic resin film of a thermosensitive stencil sheet in which the resin film is laminated with a porous thin paper, to form imagewise perforations in the stencil sheet. Japanese Laid-Open Patent Publication No. 6-127106 discloses a method for forming a stencil plate in which laser light irradiates the front side of a thermosensitive stencil sheet to form imagewise perforations therein utilizing heat which is generated with the laser light in a layer of ink, which is formed on the back side of the stencil sheet and which includes a heat generating material which generates heat by absorbing the laser light. By using these methods, the drawbacks of poor resolution and adhesion of the resin film to thermal printheads are mitigated or avoided.
However, the method disclosed in Japanese Laid-Open Patent Publication No. 61-229560 has a drawback in that the resin film itself has to absorb the laser light so as to be perforated and therefore the light source is particularly limited to high-powered laser light sources which can oscillate laser light having a long wavelength not less than 1 xcexcm. Therefore, since a laser diode having a relatively low cost cannot be used for the stencil printing apparatus used for the method, the printing apparatus becomes large and manufacturing cost of the printing apparatus increases. In addition, the method disclosed in Japanese Laid-Open Patent Publication No. 6-127106 has a drawback in that the heat generated in the ink layer tends to diffuse to the layer of ink and therefore it takes relatively large energy to make perforations in a stencil sheet, resulting in decrease of the speed of making a stencil plate. Further, the method has a drawback in that the image qualities thereof deteriorate because the perforations tend to be blocked with the ink in which the vehicle therein is evaporated due to the heat generated, or in the case of an emulsion type ink, because an aqueous phase and an oil phase of the emulsion ink separate from each other due to the heat generated.
In attempting to overcome these drawbacks, thermosensitive stencil sheets have been disclosed which can absorb laser light emitted from a compact laser diode, and methods for making stencil plates using the stencil sheets have also been disclosed.
For example, Japanese Laid-Open Patent Publication No. 62-33689 discloses a thermosensitive stencil sheet in which a thermoplastic resin film including an agent capable of absorbing an electromagnetic wave is laminated with a porous material. Japanese Laid-Open Patent Publication No. 62-181149 discloses a method for preparing a stencil plate in which the stencil sheet is perforated with semiconductor laser light according. to image information. In addition, Japanese Laid-Open Patent Publication No.6-64134 discloses a method for preparing a stencil plate in which a thermosensitive stencil sheet including an agent capable of absorbing an electromagnetic wave is perforated with a laser beam wherein the ten-point mean roughness of the surface of the stencil sheet is smaller than the depth of focus of the laser beam.
However, the layers of these stencil sheets which include an electromagnetic wave absorbing agent tend to have relatively low mechanical strength. In stencil printing, a stencil plate is mechanically stressed by printing pressure when an ink is transferred to a receiving paper. Therefore, the electromagnetic wave absorbing layers having relatively low mechanical strength are easily broken. If the refuse of the layers adheres to the receiving paper, the refuse makes the resultant print images dirty. In addition, if the refuse adheres to the stencil plate, serious image defects such as background fouling (black spots) are produced. In particular, when the stencil printing is performed for a long time, the stencil plate is seriously damaged because of being subjected to the mechanical stress many times.
Although the durability of a stencil plate in continuous use is a very important property of the stencil plate, the durability of the stencil plate which is prepared by a stencil plate making method using an electromagnetic wave as a perforating medium has not been studied at all. Therefore there have been no proposals, by which the durability of the stencil plate is improved, in the case when the stencil plate is prepared using an electromagnetic wave.
Because of these reasons, a need exists for a thermosensitive stencil sheet which can produce a stencil plate by a method in which a stencil plate is prepared with a low-cost and compact stencil printing apparatus using an electromagnetic wave such as laser light and the resultant stencil plate can produce good images on a receiving material even when printing is performed for a long time.
Accordingly, an object of the present invention is to provide a thermosensitive stencil sheet which can produce a stencil plate by a method in which a stencil plate is prepared with a low-cost and compact stencil printing apparatus using an electromagnetic wave as a perforation forming medium and the resultant stencil plate can produce good images on a receiving material even when printing is performed for a long time.
Another object of the present invention is to provide a stencil printing method in which images or color images having good image qualities are formed on a receiving material using a low-cost and compact stencil printing apparatus using an electromagnetic wave as a perforation forming medium.
To achieve such objects, the present invention contemplates the provision of a thermosensitive stencil sheet including at least a thermoplastic resin film and an electromagnetic wave absorbing layer which includes an electromagnetic wave absorbing agent and a resin. The stencil sheet preferably includes a porous substrate which is laminated on the resin film, and in addition, a protective layer may be formed overlying the electromagnetic wave absorbing layer.
In another aspect of the present invention, a stencil printing method is provided which includes the steps of:
providing a thermosensitive stencil sheet which includes a thermoplastic resin film and an electromagnetic wave absorbing layer including an electromagnetic absorbing agent and a resin, and optionally includes a porous substrate and/or a protective layer;
irradiating the thermosensitive stencil sheet with laser light to form a stencil plate in which imagewise perforations are formed in the stencil sheet; and
applying an ink on one side of the stencil plate, the opposite side of which contacts a receiving material under pressure, to transfer an ink image onto the receiving material.
By repeating these steps using, for example, a yellow, a magenta, a cyan and a black ink, a full color image can be obtained.
The stencil plate is preferably wound on a peripheral surface of a drum such that the stencil plate contacts a receiving material, and is then inked by inking from the inside of the drum to form an image on the receiving material.